Thermal cracking of hydrocarbons may be carried out in the presence of steam. Steam cracking is a well-known process and is described in U.S. Pat. No. 3,641,190 and British Patent No. 1,077,918, the teachings of which are hereby incorporated by reference. In commercial practice, steam cracking is carried out by passing a hydrocarbon feed mixed with 20-95 mol. % steam through metal pyrolysis tubes located in a fuel fired furnace to raise the feed to cracking temperatures, e.g., about 1400.degree. to 1700.degree. F. and to supply the endothermic heat of reaction, for the production of products including unsaturated light hydrocarbons, particularly C.sub.2 -C.sub.4 olefins and diolefins, especially ethylene, useful as chemicals and chemical intermediates.
The cracked effluent may be cooled in a transfer line exchanger (TLE). That name has been used to designate a heat exchanger, connected in a transfer line between the furnace and fractionator, that is located as close as possible to the outlet of the cracking furnace in order to minimize the time that the product gas is at a high temperature beyond the outlet of the cracking furnace, so as to reduce coking and yield degradation. It may also be noted that two TLE's may be used in series in a transfer line. One may then distinguish between a primary (1.degree. ) TLE and a secondary (2.degree. ) TLE. When these terms are used, the former designates a TLE which is connected to the outlet of the cracking furnace and the latter designates a TLE which is downstream of the primary TLE and cools the cracked gas to a still lower temperature. Conventionally, the cracked gas from many reaction tubes is manifolded, passed into a cone-shaped passage, sometimes referred to as a transition piece or section, thence through a tube sheet and into the cooling tubes of a multitube shell-and-tube TLE in order to cool the gas and generate steam--see U.S. Pat. No. 3,574,781. U.S. Pat. No. 4,097,544 additionally describes a shield forming with the entrance tube sheet of a shell-and-tube TLE a chamber adapted to receive low temperature steam and release the same to mix with incoming cracked gas as a direct prequench, the shield having a conical shape for the purpose of reducing recirculation of gas and thus residence time. See also U.S. Pat. No. 3,357,485 which describes a similarly shaped device for a multitube double-pipe TLE.
When residence time and hydrocarbon partial pressure of the process gas are decreased and cracking is carried out at higher radiant coil outlet temperature, the selectivity to desirable olefins is improved. It is highly desirable to reduce pressure build-up across the exchanger and loss of heat transfer, viz., by preventing heavy components in the cracked gas from condensing and sticking to the inside wall of the tubes thereby restricting flow and impeding heat transfer. From a process point of view, not only the unfired high temperature residence time needs to be minimized, but also the pressure drop in the transfer line and TLE outside of the fire box must be reduced to improve the selectivity, because large pressure drops resulting in increased pressure and increased hydrocarbon partial pressure in the upstream pyrolysis tubes connected thereto, which adversely affects the pyrolysis reaction.
The present invention teaches to reduce pressure loss in the gas when operating in the liquid washed TLE mode.
Upon cooling the effluent from a steam cracker furnace, in particular one cracking a gas oil feed, in an exchanger, the higher boiling components condense on the tube walls and foul the exchanger. The heavier components in the cracked gas condense and because they are viscous and reactive they tend to stick to the wall and polymerize. This deposit grows and can seriously restrict exchanger tubes causing a drastic increase in the exchanger pressure drop as well as impeding heat transfer.
To overcome this problem, the use has been suggested of a recycle wash stream which flows down the inside wall of the exchanger tube and acts as a solvent for the components that condense. In order to be effective, the wash liquid must flow through each tube. To accomplish this U.S. Pat. No. 4,233,137 proposes that the wash liquid be sprayed into the vapor stream entering the exchanger so as to quench the gas. The cooled vapor-liquid mixture then flows through a grid distributor which distributes the liquid to the tubes in the exchanger. A uniform distribution of the liquid to the tubes is difficult to achieve with only a spray system so that it was necessary to provide an orifice distributor at the inlet of the exchanger to help dispense the liquid evenly to the tubes. To be effective, this distributor has to take a pressure drop in the gas of more than 1 psi. This is clearly a disadvantage because it causes higher pressures upstream of the exchanger which reduces cracking selectivity. The subject invention describes an improved way of introducing tne liquid wash to the exchanger that does not have the disadvantage of excessive heating of the wash liquid in the quench operation (which causes degradation of the wash liquid) and eliminates effluent pressure drop due to such distributor as well as due to acceleration of the wash liquid when it is injected as a spray.
U.S. Pat. No. 3,959,420 describes an apparatus for direct quench of pyrolysis gases issuing from a pyrolysis furnace in which oil is caused to overflow from a reservoir so as to coat the inner wall of a quench pipe with a film to prevent fouling. However, with the different configuration of a single quench pipe, the patent does not provide any suggestion applicable to the problem faced in liquid washing of the tubes of a multitube shell-and-tube TLE which may have hundreds of tubes. See also U.S. Pat. Nos. 3,907,661; 4,121,908 and 3,593,968.